The present invention relates to a mechanism for combining two or more mechanical input signals to provide an output signal, with means for changing the ratio of the output signal to the input signals, the range of adjustment extending down to zero output or lockout.
The mechanism of the present invention is particularly intended for use in aircraft control systems for actuating control surfaces, although its usefulness is not necessarily limited to this particular application. In some aircraft, particularly those designed for short takeoff and landing (STOL) operation, the aircraft lands and takes off at very low speed but cruises at relatively high speed. Such aircraft are often equipped with inboard and outboard elevator control surfaces, but because of the great difference in speed under different flight conditions, the aircraft would be oversensitive in the pitch axis at cruise speeds if the same large elevator deflections were used at high speed as are required for low speed landing and takeoff conditions. For this reason, the outboard elevator angular travel must be gradually decreased as the air speed increases, and should reach a locked position at a predetermined relatively high speed. The outboard elevators receive control inputs from two sources, that is, from the inboard elevators which are directly controlled by the pilot, and from a series electric command servo which is part of the stability augmentation system. A control system for the outboard elevators, therefore, must provide for combining these two input signals to provide a single output signal for operating the elevator, and must include means for changing the ratio so that as the air speed increases the ratio of output-to-input signals is decreased and reaches a zero output or lockout position at a predetermined speed. Ratio changing mechanisms have, of course, been known heretofore but no such mechanism has been available which combines all the features which are necessary or desirable for a mechanism intended for use in an aircraft control system as described.